Diamond surface acoustic wave devices

ABSTRACT

A high frequency Surface Acoustic Wave (SAW) device includes a highly oriented diamond layer adjacent a piezoelectric layer. In one embodiment, laterally spaced apart piezoelectric layers or portions confine propagation of the wave within the diamond layer. Interdigitated electrodes may be provided by electrically conductive metal lines and/or by heavily doped surface portions of the diamond layer. Undesirable reflections may be reduced by providing the piezoelectric layer with opposing ends canted at an angle from orthogonal to the axis of surface acoustic wave propagation. The surface acoustic wave device may be used as a filter, amplifier, convolver, and phase shifter. Methods for making the surface acoustic wave device are also disclosed.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices, and moreparticularly, to surface acoustic wave (SAW) devices.

BACKGROUND OF THE INVENTION

Surface Acoustic Wave (SAW) devices are semiconductor devices that usesurface acoustic waves whose energy is transmitted convergently on thesurface of a solid. In general, a SAW device includes a layer of apiezoelectric material and one or more interdigitated transducer (IDT)electrodes formed on the piezoelectric layer. The surface acoustic wavemay be excited by applying an electrical signal to an IDT electrode.Electrical signals are correspondingly generated across the opposite IDTelectrode as surface acoustic waves pass the electrode. Typicalpiezoelectric materials include bulk monocrystals of quartz, as well aslayers of LiNbO₃, LiTaO₃, AlN, or ZnO grown on a substrate.

In general, the active frequency (f) of a surface acoustic wave deviceis determined by the formula f=v/λ, where λ is the wavelength and v isthe propagation velocity of the surface acoustic wave in thepiezoelectric material. The wavelength λ is dependent on the spacingfrequency of the interdigitated electrodes and the crystal orientationof the surface of the material through which the wave passes. Typicalpropagation velocities v for exemplary materials are as follows: 3500m/sec to 4000 m/sec for a monocrystalline LiNbO₃ layer, and 3300 m/secto 3400 m/sec for a monocrystalline LiTaO₃ layer. The propagationvelocity v is relatively high at approximately 3000 m/sec for a ZnO filmon a glass substrate.

The active frequency f can be increased either by increasing thepropagation velocity v or by decreasing the wavelength λ. Unfortunately,the propagation velocity is restricted by the material properties of thepiezoelectric layer. The wavelength λ, which is determined by the width,spacing, and arrangement of the IDT electrodes, is limited by the lowerlimits of existing processing technologies. In a typical interdigitatedelectrode having an array of alternating equally spaced electrodefingers with a common width w and a common spacing s, for example, thewavelength is determined by the formula λ=2s+2w. Other electrodearrangements will have other relationships between the wavelength,electrode width, and electrode spacing.

Submicron geometries may be difficult to fabricate using conventionalmaterials, and long term reliability is limited by metal migrationeffects. For example, many existing optical lithography technologiescannot be used to fabricate a line/groove structure having a width ofless than 0.8 μm. In addition, a narrower line width lowers thefabrication yield. For these reasons, the maximum frequency of manyexisting SAW devices in practical use is approximately 900 MHz.

A surface acoustic wave device having interdigitated electrodes on aLiNbO₃ substrate may have a surface acoustic wave velocity of 4003.6m/s, a coupling coefficient of 5.57%, and a frequency temperaturecoefficient of -72 ppm/K, for example. In a device having alternatingequally spaced interdigitated electrodes with 1 μm wide electrodes and 1μm spaces between electrodes, the frequency will be approximately 1 GHz.In order to achieve a 2.5 GHz device, the electrodes would need to havea width and spacing of approximately 0.4 μm.

In the case of SAW devices including a piezoelectric film on asubstrate, plural surface acoustic waves are excited if the soundvelocity of the substrate is different than the surface acoustic wavevelocity of the piezoelectric film. These surface acoustic waves arecalled zeroth mode waves, first mode waves, second mode waves, etc.according to the order of increasing velocity. The velocities of allmodes depend on the substrate, as well as the piezoelectric film. Theuse of substrates having higher sound velocities results in highervelocities for all modes of the surface acoustic wave in the device.That is, the surface acoustic wave velocity increases in proportion tothe sound velocity of the substrate.

A multilayer surface acoustic wave device is disclosed, for example, ina reference by Shiosaki et al. entitled High-Coupling and High-VelocitySAW Using ZnO and AlN Films on a Glass Substrate, and appearing in IEEETransactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol.UFFC-33, No. 3, May 1986. The SAW device disclosed by the Shiosaki etal. reference includes a borosilicate glass sheet substrate, aC-axis-oriented AlN film on the substrate, and a C-axis-orientedpolycrystalline ZnO film on the AlN film opposite the substrate.Aluminum IDT electrodes are included between the AlN and the ZnO films.With this structure, a maximum coupling coefficient of 4.37% wasreportedly obtained where the phase velocity was 5840 m/s. The frequencytemperature coefficient of this device was 21.0 ppm/°C. at 25° C. Thephase velocity of this device, however, is still relatively low.Accordingly, high frequency performance is limited.

A surface acoustic wave device having a relatively higher propagationvelocity is disclosed in U.S. Pat. No. 5,221,870 to Nakahata et al. Thepatent discloses a SAW device having a silicon semiconductor substrate,a diamond film on the substrate, a ZnO piezoelectric layer on thediamond layer, and interdigitated transducer electrodes on thepiezoelectric layer. For the diamond film, both a single crystal andpolycrystalline film are suitable. However, a monocrystalline film ismore favorable, because there is less acoustic scattering inmonocrystalline diamond as compared to polycrystalline diamond.

Diamond is a preferred material for many semiconductor devices becauseof its hardness, relatively large bandgap, high temperature performance,high thermal conductivity, and radiation resistance. Moreover, diamondis desirable for SAW devices because it has relatively large values ofacoustic velocities. See, for example, "SAW Propagation Characteristicsand Fabrication Technology of Piezoelectric Thin Film/DiamondStructure", by Yamanouchi et al., 1989 Ultrasonics Symposium, pp.351-354, 1989. Moreover, combining diamond with relatively low velocitypiezoelectric materials results in higher SAW velocities; thus, thedemands on line spacing may be reduced for a given frequency ofoperation as disclosed, for example, in "High Frequency Bandpass FilterUsing Polycrystalline Diamond", by Shikata et al., Diamond and DiamondRelated Materials, 2 (1993), pp. 1197-1202.

U.S. Pat. No. 5,235,233 to Yamamoto and entitled Surface Acoustic WaveDevice discloses a SAW device including diamond, an AlN layer on thediamond layer and IDT electrodes on the AlN layer. In anotherembodiment, an intervening layer of SiO₂ is provided between the diamondand AlN layers. High electromechanical coupling coefficients and highphase velocities are reportedly provided by the devices.

To further monitor and control the temperature of a diamond SAW device,U.S. Pat. No. 5,235,236 to Nakahata et al. and also entitled SurfaceAcoustic Wave Device discloses a thermistor formed by a semiconductingdiamond layer which, in turn, is supported on an insulating diamondlayer of the SAW device. The thermistor cooperates with a heater tocontrol the operating temperature of the SAW device.

U.S. Pat. No. 4,952,832 to Imai et al. entitled Surface Acoustic WaveDevice also discloses a SAW device including polycrystalline or singlecrystal diamond that may be used as a filter, a resonator, a delay lineor a signal processing device and a convolver. See U.S. Pat. Nos.5,221,870 and 5,160,869 both to Nakahata et al.; High-Frequency SurfaceAcoustic Wave Filter Using ZnO/Diamond/Si Structure, by Nakahata et al.,from the International Conference on the Applications of Diamond Filmsand Related Materials, pp. 361-364, (1993); and High-Frequency SurfaceAcoustic Wave Filter Using ZnO/Diamond/Si Structure, by Nakahata et al.,1992 Ultrasonics Symposium, pp. 377-380, (1992).

The device of the Nakahata et al. U.S. Pat. No. 5,221,870, for example,may have a relatively high surface acoustic wave velocity of more than10,000 m/sec. Accordingly, the high surface acoustic wave velocity vreduces the necessity of fine IDT electrodes. In particular, Nakahata etal. discloses that IDT electrodes having a line width of 1 μm and aspacing of 1 μm may produce a surface acoustic wave with a frequency ashigh as 2 GHz. However, a polycrystalline diamond film may produceacoustic scattering and may require polishing. A single crystal diamondfilm may provide better performance, however, single crystal may beexpensive and difficult to produce.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of thepresent invention to provide a surface acoustic wave device capable ofoperating at relatively high frequencies and at high temperatures.

It is another object of the present invention to provide a highfrequency surface acoustic wave device having interdigitated transducerelectrodes permitting relatively large line widths and spacings tothereby increase reliability, yield, and ease of fabrication.

It is yet another object of the present invention to provide a surfaceacoustic wave device including diamond and having high performancecharacteristics approaching those of single crystal diamond without therelatively high cost of single crystal diamond.

These and other objects, features and advantages according to thepresent invention are provided by a surface acoustic wave deviceincluding in one embodiment a highly oriented diamond layer, apiezoelectric layer on the highly oriented diamond layer, and at leastone interdigitated transducer electrode on the piezoelectric layer.Because of the relatively high wave propagation velocity of diamond, a 1μm line and space geometry, for example, may be used to fabricate SAWdevices that operate at frequencies in excess of 2.5 GHz. Higherfrequencies may be achieved by using submicron geometries.

The highly oriented diamond layer preferably includes a plurality ofside-by-side columnar diamond grains each oriented relative to oneanother and with a tilt and azimuthal misorientation of less than about8°. The highly oriented diamond is preferably formed on a nondiamondsingle crystal substrate having a relatively close lattice match todiamond, such as for example, silicon carbide, copper, nickel and alloysthereof.

The highly oriented diamond layer has advantages over conventionalpolycrystalline or single crystal diamond films. The highly orienteddiamond film is smoother than a polycrystalline film thereby reducingany need to polish the diamond film. Polishing may cause mechanical orchemical-mechanical damage to a diamond layer. In preferred embodiments,the surface roughness of the highly oriented diamond film may be lessthan 200 Å RMS.

The highly oriented diamond layer may also reduce acoustic scatteringwhen compared to a conventional polycrystalline film. This reduction inscattering may be attributed to the smoothness of the surface and thealignment of the grain boundaries. The highly oriented diamond film isalso less expensive than a single crystal diamond film. Accordingly, lowcost, large area, and well controlled mirror smooth surfaces may beobtained in surface acoustic wave devices having a highly orienteddiamond film according to one aspect of the present invention.

The interdigitated electrodes may be positioned either between thehighly oriented diamond layer and the piezoelectric layer or on thepiezoelectric layer opposite the diamond layer. A ground plane electrodemay be positioned on the piezoelectric layer opposite the interdigitatedelectrode, or it may be positioned on the diamond layer opposite thepiezoelectric layer.

Advantages may also be achieved by isolating the surface acoustic wavesto the diamond film. This isolation may be achieved by forming twolaterally spaced apart piezoelectric layer portions on a diamond layerand transmitting surface acoustic waves across the diamond layer betweenthe two piezoelectric layers. In such a device, the velocity of theacoustic wave may be greater than through both a diamond layer and apiezoelectric material layer.

Yet another aspect of the invention permits conventional metalinterdigitated transducer electrodes to be replaced or augmented by apredetermined pattern of electrically conductive highly doped surfaceportions of the diamond layer adjacent the piezoelectric layer. Thehighly doped diamond surface portions preferably comprise highly borondoped surface portions having boron concentrations of greater than about10¹⁹ atoms cm⁻³. The highly doped diamond surface portions may alsoincrease the adhesion and performance of a corresponding pattern ofmetal lines formed on the highly doped surface portions.

Undesirable reflections of surface acoustic waves may be reduced byproviding diamond and piezoelectric layers having respective opposingends which are canted at an angle from orthogonal to an axis of surfaceacoustic wave propagation. The reflections may also be further reducedby providing acoustic absorbers adjacent the ends of the device. Anacoustic absorber may also be included adjacent an end of thepiezoelectric layer. The acoustic absorber may comprise a resistor onthe piezoelectric layer, or a doped diamond resistor formed adjacent thepiezoelectric layer.

The SAW device may include first and second interdigitated electrodes onthe piezoelectric layer in laterally spaced apart relation such as todefine a SAW filter. By providing first and second interdigitatedelectrodes, an insulating layer on the piezoelectric layer positionedbetween the first and second interdigitated electrodes, a semiconductinglayer on the insulating layer, and a pair of amplification electrodes onthe semiconducting layer, the SAW device defines an amplifier.Alternately, first and second interdigitated electrodes on thepiezoelectric layer, a third electrode on the piezoelectric layerbetween the first and second interdigitated electrodes, and a fourthelectrode on the diamond layer opposite the third electrode define aconvolver or a phase-shifter.

The piezoelectric layer may be a layer of ZnO, AlN, PbZrO₃, PbTiO₃,LaZrO₃, LaTiO₃, LiTaO₃, LiNbO₃, SiO₂, Ta₂ O₅, Nb₂ O₅, BeO, Li₂ B₄ O₇,KnbO₃, ZnS, ZnSe, or GaAs. A substrate may also be included on thehighly oriented diamond layer opposite the piezoelectric layer.

A method according to the present invention is for making or fabricatinga SAW device as described above. The method preferably includes thesteps of forming a highly oriented diamond layer; forming apiezoelectric layer on the highly oriented diamond layer; and forming atleast one interdigitated electrode on the piezoelectric layer. The stepof forming the highly oriented diamond layer preferably includesforming, on a single crystal nondiamond substrate, a diamond layercomprising a plurality of side-by-side columnar diamond grains eachoriented relative to one another and with a tilt and azimuthalmisorientation of less than about 8°.

The step of forming the piezoelectric layer, in one embodiment, includesforming first and second piezoelectric layer portions on the diamondlayer in laterally spaced apart relation. Accordingly, the step offorming the interdigitated electrode comprises forming first and secondinterdigitated electrodes on respective first and second piezoelectriclayer portions. The step of forming the interdigitated electrodes inanother embodiment of the invention preferably includes forming apredetermined pattern of electrically conductive highly doped surfaceportions of the diamond layer adjacent the piezoelectric layer. The stepof forming the piezoelectric layer also preferably includes forming thepiezoelectric layer to have opposing ends canted at an angle fromorthogonal to the axis of surface acoustic wave propagation to reduceundesirable wave reflections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a surface acoustic wavedevice according to the present invention.

FIG. 2 is schematic plan view of the surface acoustic wave device asshown in FIG. 1.

FIG. 3 is a schematic cross-sectional view of another embodiment of asurface acoustic wave device according to the present invention.

FIG. 4 is a schematic cross-sectional view of yet another embodiment ofa surface acoustic wave device according to the present invention.

FIGS. 5a and 5b are photomicrographs of a highly oriented diamond layerwhich may be used in the surface acoustic wave devices according to theinvention.

FIG. 6 is a schematic cross-sectional view of a diamond surface acousticwave phase shifter or convolver according to the invention.

FIG. 7 is a schematic cross-sectional view of a diamond surface acousticwave amplifier according to the invention.

FIG. 8 is a schematic cross-sectional view of a diamond surface acousticwave device having spaced apart piezoelectric layer portions accordingto the invention.

FIG. 9 is schematic plan view of a diamond surface acoustic wave devicehaving canted opposing ends according to the invention.

FIGS. 10-13 are schematic cross-sectional views of diamond surfaceacoustic wave devices having various arrangements of acoustic waveabsorbers.

FIG. 14 is a schematic cross-section view of a diamond surface acousticwave device including a thermistor and a heating resistor according tothe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. Like numbers refer to like elements throughout and primenotation is used to indicate like elements in various embodiments.

One embodiment of a surface acoustic wave device according to theinvention is the SAW filter 20 as illustrated in FIGS. 1 and 2. The SAWfilter 20 includes a substrate 21, a highly oriented diamond layer 22 onthe substrate, a piezoelectric layer 23 on the diamond layer, and a pairof interdigitated transducer electrodes 24 on the piezoelectric layer.Alternately, the substrate may be removed from the diamond layer 22. Inthe illustrated embodiment, the IDT electrodes 24 are formed from apatterned conductive metal layer as would be readily understood by thoseskilled in the art. Each IDT transducer electrode 24 includes aplurality of spaced apart metal fingers 25. Each finger 25, in turn, hasa width w, and the spacing between adjacent fingers is indicated by s.Contact pads 26 (FIG. 2) facilitate external connection to theinterdigitated electrodes 24.

In the illustrated configuration, the frequency of the SAW filter 20 isdetermined by the formula f=v/λ. With a ZnO piezoelectric layer 23 on adiamond layer 22, in turn, positioned on a silicon substrate 21, forexample, v=10,200 m/s, and λ=2s+2w. Accordingly, if s=1 μm and w=1 μm,the frequency will be approximately 2.5 GHz. This high frequency SAWfilter 20 may be readily manufactured using conventional high capacityphotolithography equipment to form the IDT electrodes 24. SAW devicescapable of operating at still higher frequencies may be obtained byusing submicron geometries.

In alternate configurations, the interdigitated transducer electrodesmay transmit surface acoustic waves either unidirectionally orbidirectionally. As shown in FIG. 2, alternating electrode fingers maybe connected to opposite side portions of the electrode. Alternately,two or more adjacent electrode fingers may be connected to the same sideportion of the electrode. Furthermore, the width of fingers and thespacings between fingers may be varied. Accordingly, variousconfigurations of interdigitated electrodes will be readily appreciatedby one having skill in the art. The SAW device may also include floatingfingers and/or reflection gratings comprising shallow grooves,pedestals, or thin-film metal strips as would be readily understood bythose skilled in the art.

The piezoelectric layer 23 may preferably comprise ZnO, AlN, PbZrO₃,PbTiO₃, PbZrO₃, PbTiO₃, LaZrO₃, LaTiO₃, LiTaO₃, LiNbO₃, SiO₂, Ta₂ O₅,Nb₂ O₅, BeO, Li₂ B₄ O₇, KNbO₃, ZnS, ZnSe, or GaAs.

As shown in FIG. 3, in another embodiment of the SAW filter 20', theinterdigitated electrodes 24' are positioned on the piezoelectric layer23' opposite the diamond layer 22'. In addition, respective ground planeelectrodes 27' are illustratively positioned between the diamond layer22' and the substrate 21' The electrodes may comprise doped portions ofthe diamond layer, conductive portions of the substrate, or separateconductive layers.

Yet another embodiment of the SAW filter 20" is shown in FIG. 4. In thisembodiment, the positions of the interdigitated electrodes 24" and theground electrodes 27" are changed from FIG. 3. The interdigitatedelectrodes 24" are between the diamond layer 22" and the piezoelectriclayer 23", and the ground plane electrodes are on the piezoelectriclayer opposite the diamond layer.

FIG. 4 also illustrates another aspect of the present invention, whereinthe SAW filter 20" includes highly doped surface portions 22a" of thediamond layer 22". These highly doped surface portions 22a" arepreferably doped with a dopant, such as boron, to a concentration of notless than about 10¹⁹ cm⁻³. These highly doped diamond portions may beachieved by implanting boron and etching graphitized diamond asdisclosed in U.S. Pat. No. 5,254,862 to Das et al. entitled, "DiamondField Effect Transistor with a Particular Boron Distribution Profile",the entire disclosure of which is incorporated herein by reference. Thehighly doped surface portions 22a" provide better ohmic contact betweenthe diamond layer 22" and the metal fingers 25". Considered in slightlydifferent terms, the highly doped surface portions 22a" form a part ofthe interdigitated electrodes 24" Moreover, the highly doped surfaceportions 22a" may be used alone as the interdigitated electrodes,without requiring the illustrated electrically conductive fingers 25".

The diamond layer for each of the SAW filter embodiments may preferablybe provided by a highly oriented diamond layer having a plurality ofside-by-side columnar diamond grains oriented relative to one anotherand with a tilt and azimuthal misorientation of less than about 8°.Sincethe surface of this highly oriented diamond layer is smooth, extensivepolishing will not be required, and the fabrication process issimplified. In addition, diamond has the advantages of a large thermalconductivity, a low dielectric constant, a high Young's modulus, and ahigh SAW velocity.

Diamond's high thermal conductivity results in improved thermalmanagement allowing a diamond device to handle more power than acomparable conventional semiconductor device. A low dielectric constantmay help in impedance matching of surface acoustic wave devices intoelectronic circuits. Furthermore, diamond is inert to most chemicals andchemical reactions making it ideally suited for use in hazardousenvironments.

FIG. 5a is a photomicrograph illustrating a highly oriented diamondlayer 22, wherein not only are the exposed faces parallel, but they arealso rotationally aligned thereby permitting grain boundaries tosubstantially disappear with continued diamond growth as shown in FIG.5b. The highly oriented diamond layer 22 is further described incopending U.S. patent applications Ser. No. 08/035,643, filed on Mar.23, 1993 entitled Microelectronic Structures on a Nondiamond Substrateand Associated Fabrication Methods and Ser. No. 08/166,408, filed onJan. 13, 1993 entitled Electrochemical Cell Having Diamond Electrode AndMethods For Making Same, both assigned to the assignee of the presentinvention, the entire disclosures of which are hereby incorporatedherein by reference.

The highly oriented diamond layer 22 includes a plurality ofside-by-side columnar single crystal diamond grains extending outwardlyfrom the nondiamond substrate 21. Substantially all of the columnarsingle crystal diamond grains are preferably oriented with a tilt andazimuthal misorientation of less than about 8°, and more preferably,less that about 5° relative to the single crystal nondiamond wafer. Thediamond nucleation site density, or concentration is also relativelyhigh, that is, greater than about 10⁴ /cm² and, more preferably, greaterthan about 10⁵ /cm². The method for making the highly oriented diamondlayer includes carburizing the wafer surface, nucleating the carburizedwafer, and growing diamond onto the nucleated wafer to favor growth ofthe (100)-oriented face. In addition, a carbide interfacial layer ispreferably formed between the highly oriented diamond layer 22 and thenondiamond substrate 21 (not shown).

Nucleating the carburized wafer face preferably includes exposing thewafer face to a carbon containing plasma while electrically biasinganother diamond layer adjacent the wafer face and which is also exposedto the plasma. The electrical biasing is preferably carried out at apeak absolute value of not less than about 250 volts negative withrespect to ground. The electrical bias supplied may be pure DC, pulsedDC, alternating current (AC 50 or 60 Hz), or radio frequency (RF).

Without wishing to be bound thereto, applicants theorize that theadjacent diamond layer contributes to the enhancement of diamondnucleation either of two mechanisms. First, it is theorized that thediamond is chemically transported from the adjacent diamond film to thewafer. In other words, it is possible that the diamond is being movedfrom the diamond film adjacent the wafer face via an etching anddeposition process. A second theory is that increased gas phasedissociation is caused by electron emission from the diamond film andthat a higher concentration of dissociated hydrocarbons are beingcreated by this electron dissociation process.

Exposing both the nondiamond wafer and the adjacent diamond layer to thecarbon-containing plasma preferably includes exposing both to thecarbon-containing plasma having an atomic percentage of carbon of notmore than about 0.3 atomic percent, such as provided by a methane gasplasma mixture having a percentage of methane of not more than about 5percent by weight. The face of the wafer may also preferably beoptically monitored and the electrical biasing discontinued responsiveto a change in the substrate indicative of the start of growth of adiamond film on the wafer. For example, laser reflection interferometryor optical pyrometry may be used to monitor the face of the substrate.

Diamond is preferably deposited onto the wafer while controllingprocessing conditions to favor growth of diamond having a (100)-orientedouter face. Other orientations for the outer face are also possible toachieve and may be desirable in certain applications. For example, (110)and (111) orientations may also be readily obtained by controllingdiamond growth conditions to favor these orientations as would bereadily understood by those skilled in the art.

The highly oriented diamond layer 22 permits the use of a relativelyinexpensive wafer or substrate material, such as silicon, on which ahigh quality diamond layer may be formed. Diamond may be furtherdeposited onto the highly oriented diamond layer, as would be readilyappreciated by those skilled in the art, until a single crystal surfacemorphology were approached by outer portions of the diamond layer asshown in FIG. 5b. In FIG. 5b, a highly oriented diamond layer 28 isillustrated in a photomicrograph taken from a prospective view showingboth a cross-section 29 and an upper surface 30 of the layer. Thesurface 30 of this layer approaches a single crystal morphology.

The highly oriented diamond layer also has the advantage of reducingacoustic scattering because of the alignment of grain boundaries and thesmoothness of the surface. The alignment of the diamond crystals reducesany need to polish the surface of the diamond film. The alignment of thecrystals also reduces voids in the diamond layer.

Referring now to FIG. 6, there is illustrated a surface acoustic wavedevice 40 which may be used as either a phase-shifter or a convolver.This surface acoustic wave device includes a substrate 41, a diamondlayer 42, a piezoelectric layer 43, interdigitated transducer electrodes44, and first and second electrically conductive layers 48a and 48b. Aswill be readily understood by those having skill in the art, theinterdigitated electrodes 44 may be located on the piezoelectric layeropposite the diamond layer. In addition, the interdigitated electrodes44 may alternately be provided by highly doped surface portions of thediamond layer 42. The device may also include ground plane electrodesnot shown in this illustration. In the illustrated embodiment, thesubstrate 41 is also electrically conductive and cooperates with theelectrically conductive layer 48b to define a second electrode.

When configured as a phase-shifter, the surface acoustic wave device 40of FIG. 6 operates as follows. A voltage is applied between theelectrodes or electrically conductive layers 48a and 48b. An electricfield induced in the semiconductor substrate 41 and diamond layer 42changes the state of carriers. Thus, the capacitance between theelectrodes is varied by the electric field. The change of thecapacitance varies the deformation-voltage property of the piezoelectriclayer 43. If the first or gate electrode 48a is positively biased withrespect to the lower electrode 48b, the carriers in the semiconductorsubstrate increase and the semiconductor loses its resistivity.Accordingly, the insulating diamond alone contributes to the capacitancebetween the electrodes 48a and 48b, and the capacitance increases. Ifthe gate electrode is negatively biased with respect to the lowerelectrode, the number of carriers in the semiconductor decrease, and thecapacitance decreases. Accordingly, the phase velocity of a surfaceacoustic wave propagating in the piezoelectric layer 43 changes inresponse to the change of capacitance.

When the semiconductor device 40 is configured as a convolver, bothinterdigitated electrodes 44 generate surface acoustic waves. Eachsurface acoustic wave propagates from a respective interdigitatedelectrode to the central portion of the piezoelectric layer 43. The twosurface acoustic waves collide with each other in the central portion.The collision of the two waves induces a voltage across the electrodesprovided by the electrically conductive layers 48a and 48b which isproportional to the product of the wave functions of the two surfaceacoustic waves. The output voltage across electrodes is in proportion tothe convolution of the two surface acoustic waves. The frequency of theoutput wave is the sum of the frequencies of the two input waves.

FIG. 7 illustrates a surface acoustic wave amplifier 50 including asubstrate 51, a diamond layer 52, a piezoelectric layer 53, andinterdigitated electrodes 54. In addition, the amplifier 50 includes aninsulating layer 60 on the piezoelectric layer 53 between theinterdigitated electrodes 54. A semiconducting layer 61 is on theinsulating layer 60, and a pair of amplification electrodes 62 are, inturn positioned on the semiconducting layer 61. As will be understood bythose having skill in the art, the amplifier may also include groundplane electrodes not shown.

An electrical signal for controlling the amplification is applied acrossthe amplification electrodes 62 to induce an electric field in thesemiconductor layer 61. The electrical signal is preferably a pulsedsignal to thereby induce a pulsed electric field in the semiconductorlayer 61. An alternating current (AC) electrical signal is applied toone of the two interdigitated electrodes 54 to generate surface acousticwaves in the piezoelectric layer 53. The frequency of the surfaceacoustic wave is related to the frequency of the AC signal which isapplied to the interdigitated electrode.

The surface acoustic waves propagate in the piezoelectric layer 53 fromone interdigitated electrode to the other interdigitated electrode toproduce a periodically varying electric field. The carriers in thesemiconductor film 61 are affected by this electric field as follows. Ifthe carrier velocity in the semiconductor layer 61 is higher than thesurface acoustic wave velocity, the carriers are pulled backward anddecelerated by the electric field thereby amplifying the surfaceacoustic wave. If the carrier velocity is lower than the surfaceacoustic wave velocity, the carriers are pulled forward and acceleratedby the electric field induced by the surface acoustic wave therebyattenuating the surface acoustic wave. The rate of amplification orattenuation is controlled by the carrier velocity which is determined bythe pulsed electrical signal applied to the amplification electrodes.

Another embodiment of the surface acoustic wave filter according to theinvention is explained with reference to FIG. 8. The embodiment includesa substrate 71, a diamond layer 72, a pair of interdigitated electrodes74, and a pair of spaced apart piezoelectric layers 73a, 73b. In otherwords, a gap 79 is defined between laterally spaced apart piezoelectriclayer portions 73a, 73b. The filter may also include ground planeelectrodes not shown.

In this SAW device 70, surface acoustic waves are generated byelectrical signals applied to one or both of the interdigitatedelectrodes 74. Unlike the previously discussed embodiments where thesurface acoustic wave travels through both the piezoelectric layer andthe diamond layer, in the SAW filter 70 of FIG. 8, the wave propagatesonly through the diamond layer. The surface acoustic wave propagationcharacteristics are thus determined by the diamond layer rather than thecombination of the diamond layer and the piezoelectric layer.Accordingly, the surface acoustic waves may propagate at a highervelocity between the IDT electrodes 74.

Yet another aspect of the present invention is illustrated in FIG. 9.The illustrated surface acoustic wave filter 80 includes a piezoelectriclayer 83 overlying a diamond layer and substrate as described above withreference to FIGS. 1 and 2. The pair of IDT electrodes 84 define an axisof surface acoustic wave propagation 89 therebetween. In the illustratedembodiment of FIG. 9, the opposing ends 84a, 84b of the piezoelectriclayer are canted at an angle from orthogonal to the axis of surfaceacoustic wave propagation 89 to reduce undesirable reflections. Thesurface acoustic wave device 80 may also have respectively opposingsides 83c, 83d parallel to the axis of acoustic wave propagation 89.

Other embodiments are also contemplated including only one end cantedfrom orthogonal to the axis of propagation 89. In addition, the opposingends 83a, 83b may be arranged to define a trapezoid rather than beingparallel as in the illustrated embodiment of FIG. 9. While theinterdigitated electrodes 84 are shown on the surface of thepiezoelectric layer 83 opposite the diamond layer, the IDT electrodesmay also be positioned between the diamond layer and the piezoelectriclayer, or may be provided by highly doped diamond portions as discussedabove.

FIGS. 10-13 illustrate surface acoustic wave devices having variousconfigurations of acoustic wave absorbers to reduce undesirablereflections. The surface acoustic wave device 90 shown in FIG. 10includes a substrate 91, a diamond layer 92, piezoelectric layer 93,interdigitated electrodes 94, and an acoustic absorber 99 adjacent eachend of the piezoelectric layer. In FIG. 11, the SAW filter 90' includesacoustic absorbers 99' on the diamond layer 92' and embedded in thepiezoelectric layer 93'. In FIG. 12, the acoustic absorbers 99" arelocated on the diamond layer 92" at the ends of the piezoelectric layer93". In FIG. 13, the acoustic absorbers 99'" are in the diamond layer92'" as may be formed by a doped portion of the diamond layer defining aresistor as would be readily understood by those skilled in the art. Inthe other embodiments, each absorber 99 may comprise a resistor such as,for example, a metal resistor. In all of the embodiments illustrated inFIGS. 10-13, the acoustic absorber 99 reduces surface acoustic wavereflections by electrically terminating the end of the piezoelectriclayer with a resistor.

Yet another aspect of the invention having a substrate 101, a diamondlayer 102, interdigitated electrodes 104 and a piezoelectric layer 103is illustrated in FIG. 14. A heating resistor 110 may be readily formedin the diamond layer 102 by ion implantation, for example. Moreover, thegenerated heat is readily conducted by the diamond layer 102. Inaddition, a semiconducting diamond thermistor 111 may also be formed tosense the temperature of the SAW device 100. The diamond resistor 110and thermistor 111 may thus be used to maintain the device 100 at apredetermined temperature as would be readily understood by thoseskilled in the art.

A method according to the present invention is for making or fabricatinga SAW device, such as described above with reference to FIGS. 1 and 2.The method preferably includes the steps of forming a highly orienteddiamond layer 22 (FIGS. 5a and 5b); forming a piezoelectric layer 23 onthe highly oriented diamond layer; and forming at least oneinterdigitated electrode 24 on the piezoelectric layer. The step offorming the highly oriented diamond layer 22 preferably includesforming, on a single crystal nondiamond substrate 21, a diamond layercomprising a plurality of side-by-side columnar diamond grains eachoriented relative to one another and with a tilt and azimuthalmisorientation of less than about 8°. The substrate may be removed fromthe diamond layer by chemical etching or other methods known to thosehaving skill in the art.

The step of forming the piezoelectric layer, in one embodiment as shownin FIG. 8, includes forming first and second piezoelectric layerportions 73a, 73b on the diamond layer 72 in laterally spaced apartrelation. Accordingly, the step of forming the interdigitated electrodescomprises forming first and second interdigitated electrodes 74 onrespective first and second piezoelectric layer portions.

As shown in FIG. 4, the step of forming the interdigitated electrodes24"in another embodiment of the invention preferably includes forming apredetermined pattern of electrically conductive highly doped surfaceportions 22a" of the diamond layer 22" adjacent the piezoelectric layer23".

Referring again to FIG. 9, the step of forming the piezoelectric layer83 also preferably includes forming the piezoelectric layer to haveopposing ends 83a, 83b canted at an angle from orthogonal to the axis ofsurface acoustic wave propagation 89 to reduce undesirable wavereflections.

A plurality of SAW devices may be manufactured by selective depositionof diamond in an array on a large area nondiamond wafer. The individualSAW devices may then be readily produced by dicing the nondiamond waferalong the lines as disclosed in U.S. Pat. No. 5,066,938 to Kobashi etal. and incorporated herein, in its entirety, by reference.

As would be readily understood by those skilled in the art, features ofthe various SAW device embodiments described above may be readily usedin other embodiments. For example, the highly oriented diamond may beused in each of the devices including the SAW filter, chemical sensor,phase shifter, convolver, and amplifier. Other signal processingdevices, such as delay lines, and resonators are also contemplated bythe invention. Accordingly, many modifications and other embodiments ofthe invention will come to one skilled in the art having the benefit ofthe teachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is to be understood that the invention is not tobe limited to the specific embodiments disclosed, and that modificationsare intended to be included within the scope of the appended claims.

That which is claimed:
 1. A surface acoustic wave device comprising:ahighly oriented diamond layer comprising a plurality of side-by-sidecolumnar diamond grains each oriented relative to one another and with atilt and azimuthal misorientation of less than about 8°; a piezoelectriclayer on said highly oriented diamond layer; and at least oneinterdigitated electrode on said piezoelectric layer; wherein saidpiezoelectric layer comprises first and second portions in laterallyspaced apart relation, and wherein said at least one interdigitatedelectrode comprises first and second interdigitated electrodes onrespective first and second piezoelectric layer portions.
 2. A surfaceacoustic wave device according to claim 1 further comprising a singlecrystal nondiamond substrate on said highly oriented diamond layeropposite said piezoelectric layer.
 3. A surface acoustic wave deviceaccording to claim 1 wherein said at least one interdigitated electrodeis positioned between said highly oriented diamond layer and saidpiezoelectric layer.
 4. A surface acoustic wave device according toclaim 1 wherein said at least one interdigitated electrode is positionedon said piezoelectric layer opposite said highly oriented diamond layer.5. A surface acoustic wave device comprising:a highly oriented diamondlayer comprising a plurality of side-by-side columnar diamond grainseach oriented relative to one another and with a tilt and azimuthalmisorientation of less than about 8°; a piezoelectric layer on saidhighly oriented diamond layer; and at least one interdigitated electrodeon said piezoelectric layer wherein said at least one interdigitatedelectrode comprises a predetermined pattern of electrically conductivehighly doped surface portions of said diamond layer adjacent saidpiezoelectric layer.
 6. A surface acoustic wave device according toclaim 5 wherein said at least one interdigitated electrode comprises apair of interdigitated electrodes on said piezoelectric layer adjacentopposing ends thereof, wherein said pair of interdigitated electrodesdefine an axis of surface acoustic wave propagation therebetween, andwherein said piezoelectric layer has opposing ends canted at an anglefrom orthogonal to the axis of surface acoustic wave propagation toreduce, undesirable wave reflections.
 7. A surface acoustic wave devicecomprising:a highly oriented diamond layer comprising a plurality ofside-by-side columnar diamond grains each oriented relative to oneanother and with a tilt and azimuthal misorientation of less than about8°; a piezoelectric layer on said highly oriented diamond layer; atleast one interdigitated electrode on said piezoelectric layer; and anacoustic wave absorber adjacent at least one end of said piezoelectriclayer wherein said acoustic wave absorber comprises a resistor on saidpiezoelectric layer and wherein said resistor comprises a doped diamondsurface portion which is positioned on said piezoelectric layer so thatsaid doped surface portion of said diamond layer defining a resistorelectrically terminates said piezoelectric layer thereby reducingsurface acoustic wave reflections.
 8. A surface acoustic wave devicecomprising:a highly oriented diamond layer comprising a plurality ofside-by-side columnar diamond grains each oriented relative to oneanother and with a tilt and azimuthal misorientation of less than about8°; a piezoelectric layer on said highly oriented diamond layer; atleast one interdigited electrode on said piezoelectric layer whereinsaid at least one interdigitated electrode comprises first and secondinterdigitated electrodes on said piezoelectric layer in laterallyspaced apart relation; an insulating layer on said piezoelectric layerpositioned between said first and second interdigitated electrodes; asemiconducting layer on said insulating layer opposite saidpiezoelectric layer; and a pair of amplification electrodes on saidsemiconducting layer so that said surface acoustic wave device defines asurface acoustic wave amplifier.
 9. A surface acoustic wave devicecomprising:a highly oriented diamond layer comprising a plurality ofside-by-side columnar diamond grains each oriented relative to oneanother and with a tilt and azimuthal misorientation of less than about8°; a piezoelectric layer on said highly oriented diamond layer;. atleast one interdigitated electrode on said piezoelectric layer whereinsaid at least one interdigitated electrode comprises first and secondinterdigitated electrodes on said piezoelectric layer in laterallyspaced apart relation; a third electrode on said piezoelectric layerbetween said first and second interdigitated electrodes; and a fourthelectrode on said diamond layer opposite said third electrode so thatthe surface acoustic wave device defines one of a surface acoustic waveconvolver and a surface acoustic wave phase-shifter.
 10. A surfaceacoustic wave device according to claim 9 wherein said at least oneinterdigitated electrode comprises a plurality of electricallyconductive lines each having a width of not less than about 1 μm and aspacing between adjacent lines of not less than about 1 μm.
 11. Asurface acoustic wave device according to claim 10 wherein each of saidelectrically conductive lines comprises one of aluminum, gold, platinum,a refractory metal, and alloys thereof.
 12. A surface acoustic wavedevice according to claim 5 wherein said piezoelectric layer comprisesone of ZnO, AlN, PbZrO₃, PbTiO₃, LaZrO₃, LaZrO₃, LiTaO₃, LiNbO₃, SiO₂,Ta₂ O₅, Nb₂ O₅, BeO, Li₂ B₄ O₇, ZnS, ZnSe, and GaAs.
 13. A surfaceacoustic wave device according to claim 5 further comprising a groundplane electrode on at least one of said highly oriented diamond layerand said piezoelectric layer.
 14. A surface acoustic wave deviceaccording to claim 5 further comprising a thermistor formed in saidhighly oriented diamond layer.
 15. A surface acoustic wave devicecomprising:a diamond layer; first and second piezoelectric layerportions on said diamond layer in laterally spaced apart relation; andfirst and second interdigitated electrodes on respective first andsecond piezoelectric layer portions; wherein said first and secondinterdigitated electrodes define an axis of surface acoustic wavepropagation therebetween, and wherein said piezoelectric layer portionshave respective opposing ends canted at an angle from orthogonal to theaxis of surface acoustic wave propagation to reduce undesirable wavereflections.
 16. A surface acoustic wave device according to claim 15wherein said diamond layer comprises a highly oriented diamond layerhaving a plurality of side-by-side columnar diamond grains each orientedrelative to one another and with a tilt and azimuthal misorientation ofless than about 8°.
 17. A surface acoustic wave device according toclaims 15 wherein said first and second interdigitated electrodes arepositioned between respective first and second piezoelectric layerportions and said diamond layer.
 18. A surface acoustic wave deviceaccording to claim 15 wherein said first and second interdigitatedelectrodes are positioned on respective first and second piezoelectriclayer portions opposite said diamond layer.
 19. A surface acoustic wavedevice according to claim 15 further comprising a ground plane electrodeon at least one of said diamond layer, and said first and secondpiezoelectric layer portions.
 20. A surface acoustic wave devicecomprising:a diamond layer; first and second piezoelectric layerportions on said diamond layer in laterally spaced apart relation; firstand second interdigitated electrodes on respective first and secondpiezoelectric layer portions; an insulating layer on said diamond layerbetween said first and second interdigitated electrodes; asemiconducting layer on said insulating layer opposite said first andsecond piezoelectric layer portions; and a pair of amplificationelectrodes on said semiconducting layer so that said surface acousticwave device defines a surface acoustic wave amplifier.
 21. A surfaceacoustic wave device comprising:a diamond layer; first and secondpiezoelectric layer portions on said diamond layer in laterally spacedapart relation; and first and second interdigitated electrodes onrespective first and second piezoelectric layer portion; wherein each ofsaid pair of interdigitated electrodes comprises a predetermined patternof electrically conductive highly doped surface portions of said diamondlayer adjacent respective piezoelectric layer portions.
 22. A surfaceacoustic wave device according to claim 21 wherein said first and secondinterdigitated electrodes each comprise a plurality of electricallyconductive lines having a width of not less than about 1 μm and aspacing between adjacent lines of not less than about 1 μm.
 23. Asurface acoustic wave device according to claim 21 further comprising asingle crystal nondiamond substrate on said highly oriented diamondlayer opposite said piezoelectric layer.
 24. A surface acoustic wavedevice comprising:a diamond layer; first and second piezoelectric layerportions on said diamond layer in laterally spaced apart relation; firstand second interdigitated electrodes on respective first and secondpiezoelectric layer portions; and an acoustic wave absorber adjacent anend of at least one of said piezoelectric layer portions.
 25. A surfaceacoustic wave device comprising:a diamond layer; first and secondpiezoelectric layer portions on said diamond layer in laterally spacedapart relation; first and second interdigitated electrodes on respectivefirst and second piezoelectric layer portions; and a thermistor formedin said diamond layer.
 26. A surface acoustic wave device comprising:adiamond layer having a predetermined pattern of electrically conductivehighly doped surface portions defining at least one interdigitatedelectrode, said predetermined pattern of highly doped surface portionsof said diamond layer having a dopant concentration of greater thanabout 10¹⁹ atoms cm⁻³ ; and a piezoelectric layer on said diamond layerand overlying said at least one interdigitated electrode.
 27. A surfaceacoustic wave device according to claim 26 wherein said diamond layercomprises a highly oriented diamond layer having a plurality ofside-by-side columnar diamond grains each oriented relative to oneanother and with a tilt and azimuthal misorientation of less than about8°.
 28. A surface acoustic wave device according to claim 27 furthercomprising a single crystal nondiamond substrate on said highly orienteddiamond layer opposite said piezoelectric layer.
 29. A surface acousticwave device according to claim 26 wherein said at least oneinterdigitated electrode comprises a pair of interdigitated electrodeson said piezoelectric layer adjacent opposing ends thereof, wherein saidpair of interdigitated electrodes define an axis of surface acousticwave propagation therebetween, and wherein said piezoelectric layer hasopposing ends canted at an angle from orthogonal to the axis of surfaceacoustic wave propagation to reduce undesirable wave reflections.
 30. Asurface acoustic wave device according to claim 26 wherein said at leastone interdigitated electrode comprises first and second interdigitatedelectrodes positioned between said piezoelectric layer and said diamondlayer in laterally spaced apart relation.
 31. A surface acoustic wavedevice according to claim 26 further comprising electrically conductivelines on said predetermined pattern of electrically conductive highlydoped diamond surface portions.
 32. A surface acoustic wave deviceaccording to claim 26 further comprising a ground plane electrode on atleast one of said diamond layer and said piezoelectric layer.
 33. Asurface acoustic wave device according to claim 26 further comprising atleast one acoustic wave absorber adjacent an end of said piezoelectriclayer.
 34. A surface acoustic wave device comprising:a diamond layer; apiezoelectric layer on said diamond layer; and at least oneinterdigitated electrode on said piezoelectric layer, said at least oneinterdigitated electrode defining an axis of surface acoustic wavepropagation, said piezoelectric layer having respective opposing endscanted at an angle from orthogonal to the axis of surface acoustic wavepropagation to reduce undesirable wave reflections wherein said at leastone interdigitated electrode comprises a predetermined pattern ofelectrically conductive highly doped surface portions of said diamondlayer adjacent said piezoelectric layer.
 35. A surface acoustic wavedevice according to claim 34 wherein said at least one interdigitatedelectrode comprises a pair of interdigitated electrodes aligned alongthe axis of surface acoustic wave propagation and positioned adjacentrespective opposing canted ends of said piezoelectric layer.
 36. Asurface acoustic wave device according to claim 34 wherein said diamondlayer also has respective opposing ends canted at an angle fromorthogonal to the axis of surface acoustic wave propagation andcorresponding to said piezoelectric layer to reduce undesirable wavereflections.
 37. A surface acoustic wave device according to claim 34wherein both said diamond and piezoelectric layers have respectiveopposing sides parallel to the axis of acoustic wave propagation.
 38. Asurface acoustic wave device according to claim 34 wherein said diamondlayer comprises a highly oriented diamond layer comprising a pluralityof side-by-side columnar diamond grains each oriented relative to oneanother and with a tilt and azimuthal misorientation of less than about8°.
 39. A surface acoustic wave device according to claim 34 whereinsaid at least one interdigitated electrode is positioned between saidhighly oriented diamond layer and said piezoelectric layer.
 40. Asurface acoustic wave device according to claim 34 wherein said at leastone interdigitated electrode is positioned on said piezoelectric layeropposite said highly oriented diamond layer.
 41. A surface acoustic wavedevice according to claim 34 wherein said at least one interdigitatedelectrode comprises a plurality of electrically conductive lines havinga width of not less than about 1 μm and a spacing between adjacent linesof not less than about 1 μm.
 42. A surface acoustic wave devicecomprising:a diamond layer having a predetermined doped surface portiondefining an electrical resistor for absorbing an acoustic wave; apiezoelectric layer on said diamond layer and overlying saidpredetermined doped surface portion so that said doped surface of saiddiamond layer defining a resistor electrically terminates saidpiezoelectric layer thereby reducing surface acoustic wave reflections;and at least one interdigitated electrode on said piezoelectric layer.43. A surface acoustic wave device according to claim 42 wherein said atleast one interdigitated electrode comprises a predetermined pattern ofelectrically conductive highly doped surface portions of said diamondlayer.
 44. A surface acoustic wave device according to claim 42 whereinsaid piezoelectric layer comprises first and second piezoelectric layerportions on said diamond layer in laterally spaced apart relation, andwherein said at least one interdigitated electrode comprises a firstinterdigitated electrode on said first piezoelectric layer portion and asecond interdigitated electrode on said second piezoelectric layerportion such that a surface acoustic wave propagates between said firstand second piezoelectric layer portions only through said diamond layerat a velocity of surface acoustic wave propagation determined only bysaid diamond layer.
 45. A surface acoustic wave device according toclaim 42 wherein said diamond layer comprises a highly oriented diamondlayer comprising a plurality of side-by-side columnar diamond grainseach oriented relative to one another and with a tilt and azimuthalmisorientation of less than about 8°.